A. Butterfly weed (Asclepias tuberosa) is easy to grow from seed.
For the quickest flowers, start seed in late August. Fresh seed requires a
period of cool, damp stratification. Provide this by placing the seed in a
bag of moist peat moss and keeping it in the refrigerator for ten to twelve
weeks. Older seed will not require this cold period to break its dormancy.
Direct seed butterfly weed outside, or grow the seedlings in a nursery bed
until the plants have five or six leaves, at which time it should be moved to
its permanent location. Give the seed proper attention while it is
germinating; though the mature plants will tolerate somewhat dry soils, the
seedlings need to be kept moist until established. Fall-grown plants will
provide moderate bloom the following year, with improved bloom in successive
seasons.

If you prefer, you can start butterfly weed in the spring. Follow the same
procedures as for fall sowing. Butterfly weed requires winter cold to bloom,
so spring-sown plants will not bloom their first summer. However, the long
summer days will allow the seedlings to grow into large plants which will
produce many blooms the following year.